Find kth character in expanded string

String

57.1% acceptance

Mar 31, 2026

You are given a string s consisting of one or more words separated by single spaces. Each word in s consists of lowercase English letters. We obtain the expanded string t from s as follows: For each word in s, repeat its first character once, then its second character twice, and so on. For example, if s = "hello world", then t = "heelllllllooooo woorrrllllddddd". You are also given an integer k, representing a valid index of the string t. Return the kth character of the string t.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn kth_character(s: String, k: i64) -> char {
    let bytes = s.as_bytes();
    let mut pos: i64 = 0;
    let mut word_pos = 0i64;
    for &b in bytes {
      if b == b' ' {
        if pos + 1 > k {
          return ' ';
        }
        pos += 1;
        word_pos = 0;
      } else {
        word_pos += 1;
        if pos + word_pos > k {
          return b as char;
        }
        pos += word_pos;
      }
    }
    unreachable!()
  }
}